There are three primary ways a short story writer can increase his income from short fiction: write and sell more to current markets, write the same amount but sell to better-paying markets, and sell reprint rights to existing stories.
Sooner or later a competent and reasonably productive short story writer will find an editor who will publish his work on a regular basis. If the writer and editor get into a rhythm of two stories a year, it's time for the writer to try for three a year, thereby increasing his income from that one market by 50%.
On the other hand, let's imagine that the two-story-a-year market pays $100/story. If the writer finds a new market that pays $200/story and he writes one story a year for each publication, he's also increased his income by 50%.
Which is the better option? From an income standpoint the options appear equal. Whether they are depends on something many short story writers don't discuss and may not even consider: income-per-hour. If it takes five hours to write a $100 story and 10 hours to write a $200 story, then it's a financial wash. The $200/story market only becomes financially worthwhile if the writer can reduce the amount of time it takes him to write a story for that market.
Two less tangible factors are important to consider as well: status and visibility. Do your peers view one market as having a higher status, and could your appearance in that publication increase your visibility among editors (especially those that might approach you with assignments), agents (especially if you desire to write books), and conference organizers (especially those that pay guest speakers).
But what if efforts to crack the higher paying market prove futile? What if the income-per-hour results in a net gain of $0? Is the effort wasted? Not for a writer who plans ahead and develops a hierarchy of submissions.
For example: For many years I wrote for several magazines that published a similar genre of fiction. The best-paying (that I sold to) paid $750/story, the next tier paid $400/story, the next tier paid $300/story, the next tier paid $100/story, and then there were a handful of publications that made only token payments. The hierarchy I developed for my submissions in that genre worked well for several years, and I sold work at all levels. (Alas, magazines change publishers, change editors, change editorial needs, or just flat go out of business and that hierarchy of submissions is now a busted chain.)
I'm currently trying to crack a new-to-me market in a genre a half-step removed from one where I'm already selling regularly. I started writing and submitting to the publication before I had established my hierarchy of submissions, and I wasn't sure what I would do with my stories if they were rejected. During the past week I've found several publications that publish similar stories and I'm in the process of developing my hierarchy of submissions for stories in that genre.
What about reprints? Selling reprints is almost like earning money for nothing. I send copies of my work to appropriate best-of-year anthologies, and I keep a watch on market reports for other reprint opportunities. Occasionally I earn more for reprint rights than I earned for the original sale.
Are there other ways for a short story writer to increase his income from short fiction? Maybe, but damned if I know what they are.